Extracted from:

Donald F. Boesch, Donald A. Anderson, Rita A. Horner, Sandra E. Shumway,Patricia A. Tester and Terry E. Whitledge. 1997. Harmful Algal Blooms in Coastal Waters: Options for Prevention, Control and Mitigation. NOAA Coastal Ocean Program, Decision Analysis Series No. 10, Special Joint Report with the National Fish and Wildlife Foundation, February 1997.

Paralytic Shellfish Poisoning

Paralytic shellfish poisoning (PSP) is a significant problem on both the east and west coasts of the U.S. Caused by several closely related species in the genus Alexandrium, PSP toxins are responsible for persistent problems due to their accumulation in filter feeding shellfish (e.g., Shumway et al. 1988), but they also move through the food chain, affecting zooplankton, fish larvae, adult fish, and even birds and marine mammals (Anderson and White 1992; Geraci et al. 1989; Shumway 1995). On the east coast, PSP is a serious and recurrent problem from Maine to Massachusetts. Connecticut, Long Island (New York) and New Jersey occasionally experience the toxin (or Alexandrium) at low levels, but these areas seem to define the southern extreme of this organism's geographic distribution. The offshore waters of George's Bank experienced a serious PSP outbreak several years ago, leading to the extended closure of the surfclam fisheries and the demise of a fledgling roe-on scallop fishery. On the west coast, PSP is a recurrent annual problem along the coasts of northern California, Oregon, Washington, and Alaska. Overall, PSP affects more coastline than any other HAB problem.

It is likely that seasonally recurring outbreaks of PSP are linked to the existence of a dormant cyst stage in the Alexandrium life history. This strategy allows the species to deposit dormant cells in sediments where they survive through harsh winter conditions and then germinate to initiate new outbreaks in subsequent years. Prior to 1972, for example, PSP was restricted to the far eastern sections of Maine ("down east") near the Canadian border. That year, however, a massive red tide causing high levels of toxicity in those areas for the first time recorded in history. Virtually every year since that event, this region has experienced PSP outbreaks, a result of the successful colonization of the area by Alexandrium spp. A similar expansion, with subsequent recurring outbreaks of Alexandrium, occurred in the Puget Sound region of Washington in the late 1970's an area with no prior history of shellfish poisoning (Nishitani and Chew 1988). Long-term climatic variability, which affects temperature, upwelling, and currents or allows cysts to survive in areas where they did not before, may be factors in such range extensions.

PSP occurs over a large geographic range, so a variety of physical mechanisms underlie the spreading of Alexandrium blooms. In southern New England , for example, localized blooms occur in small, isolated salt ponds and embayments, whereas in the southwestern Gulf of Maine, linkage has been documented between the abundance and distribution of Alexandrium and a buoyant coastal current that travels from north to south in that region (Franks and Anderson, 1992). Fresh water enters the Gulf of Maine from several large rivers in southern Maine, and the freshened coastal waters flow south in a manner that is influenced by the amount of rainfall and snowmelt, the local wind stress, and the underlying circulation of the Gulf of Maine. Toxic Alexandrium populations are closely associated with this buoyant water mass. The long distance transport of Alexandrium cells in this coastal current are responsible for PSP outbreaks in southern Maine and Massachusetts, and may even be linked to shellfish toxicity on George's Bank (Anderson and Keafer, 1992). The hydrographic mechanisms underlying PSP blooms in down-east Maine are more poorly understood than those described for the region to the southwest.

Similarly, on the west, blooms can be either localized in distribution (i.e., restricted to the inland waters of Puget Sound or the fjords of Alaska) or wide spread along the Pacific Ocean coast. In northern California, it is hypothesized that the onset of PSP toxicity is linked to the onshore movement of warm, stratified waters following the relaxation of coastal upwelling (Horner et al. in press). The relaxation events or downwelling, brought about by a change in wind speed or direction, carry established Alexandrium populations toward shore, resulting in rapid increases in toxicity in nearshore shellfish. There is currently no evidence that this also occurs in Washington or Alaska.

These are but a few of the physical mechanisms underlying PSP outbreaks in the U.S. Some areas are well-studied, and others are virtually unknown. Alexandrium blooms generally do not involve large cell accumulations that discolor the water and may be below the water surface where they are not visible. Low density populations can cause severe problems due to the high potency of the toxins produced by these species. Furthermore, Alexandrium species can grow in relatively pristine waters, and it is difficult to argue that anthropogenic nutrient inputs are stimulating the blooms. These characteristics are important when considering mitigation and control strategies.

The economic impact of these outbreaks is significant, though difficult to estimate in total. Most of the states listed above operate shellfish monitoring programs, each of which costs $100,00-200,000 per year. Estimates of the losses to shellfisheries and other seafood-related industries are few, but one listed the costs of a single PSP outbreak in Maine at $6 million (Shumway et al. 1988). Some estimates place the value of the quarantined surfclam resources on George's Bank at several million dollars per year. This resource has been closed to harvest since 1989. On the west coast, the shellfish industry in Alaska, which produced 5 million pounds of product in 1917, has been greatly reduced (except for aquaculture) as a direct result of persistent product contamination of butterclams by PSP (Neve and Reichart 1984). There is a highly restricted recreational shellfish industry since many if the state's resources remain permanently closed due to high costs associated with monitoring the state's vast coastline. The value of the sustainable, but presently unexploited, shellfish resource in Alaska is estimated to be $50 million per year (Neve and Reichart 1984). In addition to the risks of PSP from molluscs, there are PSP and domoic acid poisoning risks from consumption of Dungeness and other crabs.

UNIVERSITY OF MARYLAND College of Ag & Natural Resources Department of Veterinary Medicine Aquatic Pathobiology Center